plugin_installer. fix check update

plugin_installer/manifest.ini

 [info]
 name: Plugin Installer
 short_name: plugin_installer
-version: 0.8.1
+version: 0.8.2
 description: Install and upgrade plugins from ftp
 authors: Denis Fomin <fominde@gmail.com>
  Yann Leboulanger <asterix@lagaule.org>

plugin_installer/plugin_installer.py

@@ -93,6 +93,7 @@ class PluginInstaller(GajimPlugin):
             try:
                 to_update = []
                 con = self.ftp_connect()
+                con.cwd('plugins')
                 plugins_dirs = con.nlst()
                 for dir_ in plugins_dirs:
                     try: